Pre-expanded polypropylene resin bead and molded object obtained therefrom by in-mold foaming

ABSTRACT

The present invention provides polypropylene resin pre-expanded particles having small fluctuation in cell diameter and expansion ratio compared to in the past. The pre-expanded particles are obtained by having as a base resin, a polypropylene resin composition comprising polypropylene resin (A), a compound having a triazine skeleton and molecular weight of at most 300 per triazine skeleton unit (B) and as a random component, hydrophilic polymer (C) and inorganic filler (D) and water as the foaming agent. By using the pre-expanded particles, a polypropylene resin in-mold expanded article having decreased unevenness in color and fluctuation in weight can be obtained.

TECHNICAL FIELD

[0001] The present invention relates to a polypropylene resinpre-expanded particle and an in-mold expanded article thereof. Morespecifically, the present invention relates to a polypropylene resinpre-expanded particle which can suitably be used for preparing anin-mold expanded article of polypropylene resin used in cushioningpackaging material, returnable containers, insulating material or bumpercore of an automobile, and an in-mold expanded article made of theparticles.

BACKGROUND ART

[0002] Conventionally, a method of preparing pre-expanded particles bydispersing polypropylene resin particles in an aqueous dispersion mediumalong with a foaming agent, raising the temperature, making the pressureand temperature constant to impregnate the foaming agent in polyolefinresin particles and then discharging into low pressure atmosphere, isknown. Regarding the foaming agent, disclosed are the methods of using avolatile organic foaming agent such as propane or butane (JP-B-56-1344)and using inorganic gas such as carbon dioxide, nitrogen and air(JP-B-4-64332, JP-B-4-64334).

[0003] However, volatile organic foaming agents are expensive and socosts will rise. Also, volatile organic foaming agents such as propaneand butane have the effect of placticizing polyolefin resin and thoughhigh expansion ratio can be obtained, due to the placticizing effect,there are problems such as difficulty in controlling the expansion ratioand crystal structure of the pre-expanded particle.

[0004] When using inorganic gas such as carbon dioxide, nitrogen andair, because the inorganic gas is difficult to be impregnated into thepolyolefin resin, usually impregnation must be conducted at a highpressure of approximately 3 to 6 MPa. As a result, the impregnatingvessel for impregnating the foaming agent into the polyolefin resin musthave high pressure resistant properties and so there is the problem ofhigh facility costs.

[0005] As a method for economically preparing polyolefin resinpre-expanded particles which can suitably be used for preparing anin-mold expanded article and solves the above problems, a method ofusing the water used for the dispersion medium as a foaming agent hasbeen suggested.

[0006] A method using water as a foaming agent which has been suggestedis the method of preparing crystalline polyolefin polymer expandedparticles, which comprises dispersing crystalline polyolefin polymerparticles containing 10 to 70% by weight of an inorganic filler intowater which is the dispersion medium in a sealed vessel, impregnatingthe water which is the dispersion medium under pressure which is higherthan the saturated vapor pressure of the dispersion liquid andtemperature which is at most the melting point of the crystallinepolyolefin polymer particles, while maintaining a high pressure rangewhich is under the temperature conditions in which crystallization ofthe polymer particles progresses, and then discharging the dispersionliquid in a low pressure range (JP-B-49-2183). However, because thepre-expanded particles obtained in this way contain a great deal ofinorganic filler, cell diameter is extremely small, open cell ratiotends to become high and fusion, surface appearance and mechanicalproperties such as compressive strength when made into an in-moldexpanded article are not sufficient.

[0007] Another method which has been suggested is the method ofpreparing propylene random copolymer resin expanded particles whichcomprises dispersing propylene-ethylene random copolymer resin particlescontaining 1 to 12% by weight of ethylene into water in a sealed vessel,then introducing inorganic gas to make the pressure within the vessel atleast 5 kg/cm²G, heating to a temperature which is between the meltingpoint of the copolymer resin particles and 25° C higher than the meltingpoint and discharging the dispersion liquid in a low pressure atmosphere(Japanese Patent No. 1880374). However, according to this method, inorder to impregnate with water, high temperature and high pressureconditions must be maintained for a long period and productivity isextremely low. Also, the fluctuation in expansion ratio of the obtainedpre-expanded particles is not of a satisfactory level.

[0008] Another method which has been suggested is the method ofpreparing polyolefin resin pre-expanded particles, which comprisesdispersing polyolefin resin particles containing a hydrophilic polymerand an inorganic filler into water in a sealed vessel, then heating toat least the softening temperature of the resin particles to make watercontaining polyolefin resin particles, and then discharging thedispersion liquid in a low pressure atmosphere (for exampleJP-A-9-838048, JP-A-10-306179, JP-A-11-106576). By this method,polyolefin resin pre-expanded particles having a high expansion ratiocan be obtained under low pressure in the vessel, compared to the caseof using inorganic gas such as carbon dioxide, nitrogen and air as afoaming agent. In addition, maintenance of high temperature and highpressure for a long period is unnecessary and economical production ispossible. As for the obtained polyolefin resin pre-expanded particles,fluctuation in expansion ratio and cell diameter is small and also,fusion and surface appearance of the in-mold expanded article arefavorable.

[0009] However, regarding fluctuation in expansion ratio and celldiameter, the level of demand has become higher than in the past. Evenin the method of preparing polyolefin resin pre-expanded particles byusing polyolefin resin particles containing a hydrophilic polymer and aninorganic filler and using water for the dispersion medium as a foamingagent, cases in which fluctuation in expansion ratio and cell diameterdo not satisfy the level of demand are arising and further improvementis desired.

[0010] When fluctuation in expansion ratio is large, there is theproblem of large fluctuation in weight when made into an in-moldexpanded article. In recent years, quality standards for products havebecome more severe and in order to cut down on the man hour for weightexamination of the in-mold expanded article, pre-expanded particleshaving smaller fluctuation in expansion ratio than in the past are indemand.

[0011] In addition, further improvement is in demand as fluctuation incell diameter causes unevenness in color and damages in appearance. Inthe case of an in-mold expanded article colored by including pigment ordye, particularly in the case of an in-mold expanded article colored inblack, unevenness in color is more noticeable than in an in-moldexpanded article which is uncolored and white and therefore demand forimprovement in fluctuation of cell diameter is strong.

DISCLOSURE OF INVENTION

[0012] The object of the present invention is to provide pre-expandedarticles having little fluctuation in cell diameter and expansion ratioand decreased unevenness in color and weight fluctuation when made intoan in-mold expanded article in the case of preparing polypropylene resinpre-expanded particles by using water as a foaming agent.

[0013] As a result of devoted research to achieve this goal, it wasfound that the above objective could be achieved by using as a baseresin, a polypropylene resin composition prepared by adding a compoundhaving a specific triazine skeleton to polypropylene resin and thepresent invention was completed.

[0014] The present invention relates to a polypropylene resinpre-expanded particle comprising as a base resin a polypropylene resincomposition containing polypropylene resin (A) and a compound having atriazine skeleton and molecular weight of at most 300 per triazineskeleton unit (B).

[0015] The above polypropylene resin pre-expanded particle may alsocomprise hydrophilic polymer (C).

[0016] The above polypropylene resin pre-expanded particle preferablycontains 0.005 to 6 parts by weight of the compound (B) having atriazine skeleton and molecular weight of at most 300 per triazineskeleton unit based on 100 parts by weight of polypropylene resin (A).

[0017] The above polypropylene resin pre-expanded particle preferablycontains 0.01 to 20 parts by weight of the hydrophilic polymer (C) basedon 100 parts by weight of polypropylene resin (A).

[0018] The above polypropylene resin pre-expanded particle may alsocomprise inorganic filler (D).

[0019] The above polypropylene resin pre-expanded particle preferablycontains 0.05 to 10 parts by weight of the inorganic filler (D) based on100 parts by weight of polypropylene resin (A).

[0020] In the above polypropylene resin pre-expanded particle, thepolypropylene resin (A) is at least one member selected from the groupconsisting of ethylene-propylene random copolymer, propylene-butene-1random copolymer and ethylene-propylene-butene-1 random copolymer, or acombination thereof.

[0021] In the above polypropylene resin pre-expanded particle, thecompound (B) having a triazine skeleton and molecular weight of at most300 per triazine skeleton unit is at least one member selected from thegroup consisting of melamine, isocyanuric acid and melamineisocyanurate, or a combination thereof.

[0022] In the above polypropylene resin pre-expanded particle, thehydrophilic polymer (C) is preferably an ethylene ionomer resin obtainedby crosslinking ethylene-(meth)acrylic acid copolymer with alkali metalion.

[0023] The above polypropylene resin pre-expanded particle preferablyhas two melting peaks in the DSC curve obtained by differential scanningcalorimetry.

[0024] Furthermore, the present invention relates to an in-mold expandedarticle comprising the above polypropylene resin pre-expanded particle.

BEST MODE FOR CARRYING OUT THE INVENTION

[0025] The polypropylene resin pre-expanded particle of the presentinvention contains (A) polypropylene resin and (B) a compound having atriazine skeleton and molecular weight of at most 300 per triazineskeleton unit.

[0026] Examples of the polypropylene resin (A) used in the presentinvention are propylene homopolymer, a-olefin-propylene random copolymerand α-olefin-propylene block copolymer. These may be used alone or incombination of two or more kinds. Particularly, ethylene-propylenerandom copolymer, ethylene-propylene-butene-1 random copolymer andpropylene-butene-1 random copolymer exhibit good foaming properties andmay be suitably used.

[0027] In order to obtain pre-expanded particles excellent in foamingproperties and moldability and excellent in mechanical strength and heatresistance when made into an in-mold expanded article, the abovepolypropylene resin preferably has a usual melting point of 130° to 165°C., more preferably 135° to 155° C. and a usual melt index (hereinafterMI value) of 0.5 to 30 g/10 minutes, more preferably 2 to 20 g/10minutes.

[0028] When the melting point is lower than 130° C., heat resistance andmechanical strength tend to be insufficient. When the melting point ishigher than 165° C., melt adhesion of pre-expanded particles duringin-mold expansion molding tends to become difficult to acquire. When theMI value is less than 0.5 g/10 minutes, pre-expanded particles havinghigh expansion ratio are hard to obtain and when the MI value is greaterthan 30 g/10 minutes, the expanded cells tend to break easily and theopen cell ratio of the pre-expanded particles tends to become high.

[0029] Here, melting point is the peak temperature of the endothermicpeak in the DSC curve obtained by heating 1 to 10 mg of polypropyleneresin from 40° C. to 220° C. at a rate of 10° C./minute, then cooling to40° C. at a rate of 10° C./minute and then heating again to 220° C. at arate of 10° C./minute using a differential scanning calorimeter.Furthermore, MI value is the value when measured according to JIS K7210at a temperature of 230° C. and a load of 2.16 kg.

[0030] In the present invention, a compound having a triazine skeletonand molecular weight of at most 300 per triazine skeleton unit (B)(hereinafter, may be described as triazine compound) is used. Here,molecular weight per triazine skeleton unit is the value obtained bydividing the molecular weight by the number of triazine skeletonscontained in one molecule. The triazine compound increases the watercontent of polypropylene resin composition particles, providespolypropylene resin pre-expanded particles having high expansion ratioand has the effect of controlling fluctuation in expansion ratio andcell diameter. However, in the case that the molecular weight pertriazine skeleton unit exceeds 300, the effect of increasing watercontent and controlling fluctuation in expansion ratio and cell diameteris not sufficiently exhibited. The molecular weight per triazineskeleton unit is preferably 100 to 200.

[0031] Examples of the above triazine compound are melamine (chemicalname 1,3,5-triazine-2,4,6-triamine), ammeline(1,3,5-triazine-2-hydroxy-4,6-diamine), ammelide(1,3,5-triazine-2,4-dihydroxy-6-amine), cyanuric acid(1,3,5-triazine-2,4,6-triol), isocyanuric acid(1,3,5-triazine-2,4,6(1H,3H,5H)-trion), acetoguanamine(1,3,5-triazine-2,4-diamine-6-methyl), benzoguanamine(1,3,5-triazine-2,4-diamine-6-phenyl), tris (methyl) isocyanurate,tris(ethyl) isocyanurate, tris(butyl)isocyanurate,tris(2-hydroxyethyl)isocyanurate and melamine isocyanurate. These may beused alone or in combination of two or more kinds. Particularly,melamine, isocyanuric acid and melamine isocyanurate are preferably usedas the effect of increasing water content and controlling fluctuation inexpansion ratio and cell diameter is high.

[0032] In order to obtain a more uniform and favorable cell structure,the above triazine compound preferably has an average particle size of0.1 to 800 μm, more preferably 1 to 100 μm, and the more uniform theparticle size, the more preferable they are. Also, in order to preventcaking, 0.1 to 1% by weight of metallic soap such as magnesium stearate,barium stearate and calcium stearate may be compounded into the triazinecompound.

[0033] Furthermore, the triazine compound preferably is solid particlesat the processing temperature when preparing polypropylene resincomposition particles. When the compound has a melting point, themelting point is preferably at least 180° C. When the compounddecomposes without having a melting point, the decomposition temperatureis preferably at least 230° C.

[0034] The amount to be used of the above compound having a triazineskeleton and molecular weight of at most 300 per triazine skeleton unitis not particularly limited but usually, the upper limit of the amountto be used is preferably 6 parts by weight, more preferably 3 parts byweight, based on 100 parts by weight of the polypropylene resin. Thelower limit of the amount to be used is preferably 0.05 part by weight,more preferably 0.1 part by weight. When the amount is less than 0.05part by weight, the effect of controlling fluctuation in expansion ratioand cell diameter tends to be insufficient. When the amount is greaterthan 6 parts by weight, moldability tends to decrease as cell diameterbecomes fine and the open cell ratio rises.

[0035] The polypropylene resin pre-expanded particles of the presentinvention may also contain hydrophilic polymer (C). Examples of thehydrophilic polymer (C) to be used in the present invention are polymercontaining a carboxyl group such as ethylene-acrylic acid-maleicanhydride terpolymer, ethylene-(meth) acrylic acid copolymer or ionomerresin prepared by crosslinking ethylene-(meth)acrylic acid copolymerwith metal ion; polyamide such as nylon 6, nylon 6,6 or copolymerizednylon and thermoplastic polyester elastomer such as a block copolymer ofpolybutylene terephthalate and polytetramethylene glycol. These may beused alone or in a combination of two or more kinds. Particularly,ethylene ionomer resin prepared by crosslinking ethylene-(meth)acrylicacid copolymer with alkali metal ion such as sodium ion or potassium ionis preferable as it provides good water content and expandability.Furthermore, ethylene ionomer resin prepared by crosslinkingethylene-(meth)acrylic acid copolymer with potassium ion is particularlypreferable as it provides a larger average cell diameter.

[0036] The above hydrophilic polymer also acts as a water absorptionagent and though using the hydrophilic polymer is not mandatory, byusing it, expandability are increased, that is pre-expanded particleshaving high expansion ratio become easier to obtain. As described in“Background Art”, pre-expanded particles having high expansion ratio canalso be obtained from a polypropylene resin composition containing ahydrophilic polymer and an inorganic filler, but the fluctuation in celldiameter and expansion ratio is not small enough. By using incombination with the triazine compound, pre-expanded particles havingsufficiently small fluctuation in cell diameter and expansion ratio canbe obtained.

[0037] The amount to be used of the hydrophilic polymer depends on thetype of hydrophilic polymer and is not particularly limited. Usually,the upper limit of the amount to be used is preferably 20 parts byweight, more preferably 10 parts by weight, based on 100 parts by weightof the polypropylene resin. The lower limit of the amount to be used is0.01 part by weight, more preferably 0.1 part by weight. Furtherpreferably, the upper limit of the amount to be used is 5 parts byweight and the lower limit of the amount to be used is 0.3 part byweight. When the amount is less than 0.01 part by weight, the effect ofimproving expansion ratio tends to become small. When the amount isgreater than 20 parts by weight, decrease in heat resistance andmechanical strength tends to become large.

[0038] Furthermore, the polypropylene resin pre-expanded particles ofthe present invention may contain inorganic filler (D). Examples of theinorganic filler (D) to be used in the present invention are clay suchas talc, mica, kaoline, montmorillonite, bentonite, attapulgite,laponite and sepiolite, natural or synthetic silica, natural orsynthetic calcium carbonate, titanium oxide and zinc oxide. These may beused alone or in a combination of two or more kinds. Particularly, talchaving an average particle size of 1 to 20 μm, mica having an averageparticle size of 1 to 20 μm, swellable mica having an average particlesize of 0.1 to 10 μm, kaoline having an average particle size of 0.1 to10 μm, wet synthetic silica having an average particle size of 0.1 to 10μm and the same with modified surface, dry synthetic silica having anaverage particle size of 0.001 to 0.05 μm and the same with modifiedsurface, precipitated calcium carbonate having an average particle sizeof 0.05 to 0.5 μm and a surface modified article thereof, refinedbentonite having an average particle size of 1 to 20 μm, attapulgitehaving an average particle size of 0.05 to 0.5 μm and laponite having anaverage particle size of 10 to 200 μm provide favorable cell structureand can be used suitably.

[0039] The above inorganic filler also acts as a cell nucleating agentand assists uniform cell formation. Though using the inorganic filler isnot mandatory, by using it, expandability are increased, that is,pre-expanded particles having high expansion ratio become easier toobtain. The amount to be used of the inorganic filler is notparticularly limited, but the upper limit of the amount to be used is 10parts by weight, more preferably 5 parts by weight based on 100 parts byweight of the polypropylene resin. The lower limit of the amount to beused is preferably 0.005 part by weight, more preferably 0.01 part byweight. When the amount is greater than 10 parts by weight, mechanicalstrength and impact resistance when the pre-expanded particles are madeinto an in-mold expanded article tend to become small.

[0040] The polypropylene resin composition comprising polypropyleneresin (A), a compound having a triazine skeleton and molecular weight ofat most 300 per triazine skeleton unit (B) and containing whennecessary, hydrophilic polymer (C) and inorganic filler (D), is usuallymelted using an extruder, kneader, Banbury mixer (trademark) or roll.The composition is processed into resin particles of the desiredparticle shape such as a cylinder, elliptic cylinder, sphere, cube orrectangular parallelepiped and the weight of each resin particle is 0.2to 10 mg, preferably 0.5 to 6 mg, more preferably 1 to 5 mg. Accordingto need, additives such as a coloring agent including carbon black,antistatic agent, flame retardant, antioxidant and UV stabilizer agentmay be added.

[0041] Conventionally known methods may be used for preparing thepolypropylene resin pre-expanded particles of the present invention. Forexample, the pre-expanded particles are prepared by a method whichcomprises charging a sealed vessel with an aqueous dispersion mediumcontaining the above resin particles, dispersant and auxiliarydispersant, heating to a constant temperature while agitating to addwater to the resin particles, maintaining a constant pressure byinorganic gas (excluding carbon dioxide) such as nitrogen and air anddischarging into atmosphere in which pressure is lower than that withinthe sealed vessel, through a flow-restricting device (open orifice)having an opening of 2 to 10 mmφ. The low pressure atmosphere ispreferably maintained at a high temperature in order to raise theexpansion ratio and more preferably is maintained at 90° to 100° C. byvapor. Furthermore, using a flow-restricting device which preventsspreading of the contents within the sealed vessel when discharging, andcolliding the discharged substance into a collision board are preferableas pre-expanded particles having smaller fluctuation in expansion ratiocan be obtained. The sealed vessel to be used is not particularlylimited as long as it withstands the pressure and temperature within thevessel when preparing the pre-expanded particles and an example is anautoclave type pressure resistant vessel.

[0042] Examples of the dispersant are hardly water soluble inorganiccompounds such as basic calcium tertiary phosphate, basic magnesiumcarbonate and calcium carbonate and examples of the above auxiliarydispersant are anionic surfactants such as sodiumdodecylbenzenesulfonate, sodium n-paraffinsulfonate and sodium a-olefinsulfonate. Of these, the use of basic calcium tertiary phosphate andsodium n-paraffinsulfonate are preferable in view of obtaining gooddispersion properties. The amount to be used of the dispersant andauxiliary dispersant depends on the type thereof and the type and amountof the polypropylene resin to be used but is usually 0.1 to 3 parts byweight of dispersant, 0.0001 to 0.1 part by weight of auxiliarydispersant based on 100 parts by weight of water.

[0043] Also, in order for the dispersion properties of the resinparticles in water to become good, usually 20 to 100 parts by weight ofresin particles based on 100 parts by weight of water is preferablyused.

[0044] The pre-expanded particles obtained in this way preferably havetwo melting peaks in the DSC curve obtained by differential scanningcalorimetry. Furthermore, the difference of the two melting peaktemperatures is preferably at least 10° C. In the case that thepre-expanded particles have two melting peaks, in-mold expansionmoldability is good and an in-mold expanded article having goodmechanical strength and heat resistance can be obtained.

[0045] Here, the DSC curve of the pre-expanded particles obtained bydifferential scanning calorimetry is the DSC curve obtained when 1 to 10mg of pre-expanded particles are heated from 40° C. to 220° C. at aheating rate of 10° C./minute using a differential scanning calorimeter.

[0046] The pre-expanded particles having two melting peaks can easily beobtained by setting the temperature within the pressure vessel whenpre-expanding to a suitable value. Regarding the temperature within thepressure vessel, usually the lower limit is adjusted to a temperaturewhich is at least the softening point of polypropylene resin, which isthe main component of the polypropylene resin composition, preferably toat least melting point Tm, more preferably to at least (Tm+5)° C. andpreferably to lower than the melting completion point Te, morepreferably at most (Te−2)° C.

[0047] Here, the melting completion point is the temperature of when thebottom of the melting peak of the DSC curve, obtained by heating 1 to 10mg of polypropylene resin from 40° C. to 220° C. at a rate of 10°C./minute, then cooling to 40° C. at a rate of 10° C./minute and thenheating again to a 220° C. at a rate of 10° C./minute using adifferential scanning calorimeter, returns to the baseline position onthe high temperature side.

[0048] In the method for preparing the pre-expanded particles, heatingis conducted for 5 to 60 minutes, more preferably 10 to 30 minutes,while maintaining the adjusted temperature. When the heating time isshorter than 5 minutes, stability in the expansion ratio per batch tendsto decrease and when the heating time is longer than 60 minutes, thebatch cycle becomes longer and productivity tends to decrease.

[0049] Furthermore, the pre-expanded particles of the present inventionare preferably prepared with the pressure within the vessel set to atleast 0.6 MPa, more preferably at least 1 MPa. When the pressure withinthe vessel is less than 0.6 MPa, problems such as obstruction of theflow-restricting device (open orifice) tend to occur. The upper limit ofthe pressure within the vessel is not particularly limited butpreferably at most 5 MPa, particularly at most 4 MPa, as creating highpressure requires high facility costs and utility costs.

[0050] The pre-expanded particles obtained in this way preferably havewater content of 1 to 10%. When the water content is less than 1%,obtaining pre-expanded particles having high expansion ratio tends to bedifficult and when the water content is greater than 10%, shrinkage ofthe pre-expanded particles tends to become large due to coagulation ofwater after expansion.

[0051] The polypropylene resin pre-expanded particles of the presentinvention obtained in this way can be made into an in-mold expandedarticle by a conventionally known method. For example, methods which maybe used are (1) a method which comprises impregnating the pre-expandedparticles with inorganic gas by pressurizing with inorganic gas, therebyapplying a pre-determined pressure into the particle, filling into amold and thermally melting by vapor, (2) a method which comprisespressing the pre-expanded particles by gas pressure to fill into a moldand thermally melting by vapor utilizing the resilience of the particlesand (3) a method which comprises filling the pre-expanded particles intoa mold without any pre-treatment and thermally melting by steam.

[0052] The following methods are preferable embodiments of the presentinvention. 100 parts by weight of polypropylene resin (A) having amelting point of 130° C. to 165° C. and a MI value of 0.5 to 30 g/10minutes, 0.05 to 6 parts by weight of isocyanuric acid, melamine ormelamine isocyanurate as the compound having a triazine skeleton andmolecular weight of at most 300 per triazine skeleton unit (B) and 0 to10 parts by weight of talc as the inorganic filler (D) are mixed. Themixture is extruded in strands by an extruder and after cooling, thestrands are cut into cylindrical resin particles of 1 to 5 mg. Thecompound having a triazine skeleton and molecular weight of at most 300per triazine skeleton unit (B) and inorganic filler (D) are preferablyadded as a master batch prepared in advance. The autoclave pressurevessel was charged with 100 to 500 parts by weight of water, 0.1 to 15parts by weight of basic calcium tertiary phosphate as the dispersantand 0.0001 to 0.5 part by weight of sodium n-paraffinsulfonate as theauxiliary dispersant based on 100 parts by weight of the resinparticles. This is heated to a constant temperature of Tm to Te° C. andafter pressurizing to a constant pressure of 1 to 3 MPa by air,pre-expanded particles are prepared by discharging into vapor atmosphereof 90° to 100° C., through a flow-restricting device having an openingof 2 to 10 mmφ.

[0053] Hereinafter, the present invention is explained in detail bymeans of Examples and Comparative Examples, but is not limited thereto.

[0054] The composition of the polypropylene resin composition ofExamples 1 to 26 and Comparative Examples 1 to 4 is shown in Table 1 andthe expansion conditions and properties of the obtained pre-expandedparticles are shown in Table 2.

EXAMPLE 1

[0055] Ethylene-propylene random copolymer (melting point 145° C.,melting completion point 160° C., MI value 10 g/10 minutes) was used asthe polypropylene resin and 0.03% by weight of melamine (product name:Melamine, available from BASF Ltd., decomposition point: 300° C.) havinga molecular weight of 126 per triazine skeleton unit as the triazinecompound and 2.5% by weight of carbon black as the coloring agent werecompounded so that the total equals 100% by weight. The mixture was meltmixed in a 50 mmφ single screw extruder, extruded into strands from acylindrical die of 2.2 mmφ in diameter, cooled with water and cut with acutter to obtain resin particles of 1.8 mg/particle.

[0056] A 0.35 m³ autoclave was charged with 100 parts by weight of theobtained resin particles (65 kg), 200 parts by weight of water, 0.5 partby weight of basic calcium tertiary phosphate and 0.01 part by weight ofsodium n-paraffinsulfonate and while agitating, the contents of theautoclave were heated to the inner temperature shown in Table 2. Then,the pressure within the autoclave was increased by compressed air to thepressure inside the vessel shown in Table 2 and the temperature withinthe vessel was maintained for 30 minutes. A valve located at the lowerpart of the autoclave was opened and the contents of the autoclave weredischarged into saturated vapor atmosphere of 100° C. through an openorifice of 3.6 mmφ to obtain pre-expanded particles.

[0057] As properties of the obtained pre-expanded particles, expansionratio, the number of melting peaks in the DSC curve obtained bydifferential scanning calorimetry, open cell ratio, average celldiameter, fluctuation in cell diameter, fluctuation in expansion ratioand water content were measured. The results are shown in Table 2.

EXAMPLES 2 TO 7

[0058] Pre-expanded particles were obtained in the same manner as inExample 1 except that the amount added of melamine (product name:Melamine, available from BASF Ltd.) was the amount shown in Table 1 andproperties were measured. Expansion conditions and results are shown inTable 2.

EXAMPLE 8

[0059] Ethylene-propylene random copolymer (melting point 145° C.,melting completion point 160° C., MI value 10 g/10 minutes) was used asthe polypropylene resin and 0.08% by weight of melamine (product name:Melamine, available from BASF Ltd., decomposition point: 300° C.) havinga molecular weight of 126 per triazine skeleton unit as the triazinecompound, 0.15% by weight of talc (average particle size 8 μm) as theinorganic filler and 2.5% by weight of carbon black as the coloringagent were compounded so that the total equals 100% by weight. Themixture was melt mixed in a 50 mmφ single screw extruder, extruded intostrands from a cylindrical die of 2.2 mmφ in diameter, cooled with waterand cut with a cutter to obtain resin particles of 1.8 mg/particle.

[0060] A 0.35 m³ autoclave was charged with 100 parts by weight of theobtained resin particles (65 kg), 200 parts by weight of water, 0.5 partby weight of basic calcium tertiary phosphate and 0.01 part by weight ofsodium n-paraffinsulfonate and while agitating, the contents of theautoclave were heated to the inner temperature shown in Table 2. Then,the pressure within the autoclave was increased by compressed air to thepressure inside the vessel shown in Table 2 and the temperature withinthe vessel was maintained for 30 minutes. A valve located at the lowerpart of the autoclave was opened and the contents of the autoclave weredischarged into saturated vapor atmosphere of 100° C. through an openorifice of 3.6 mmφ to obtain pre-expanded particles.

[0061] As properties of the obtained pre-expanded particles, expansionratio, the number of melting peaks in the DSC curve obtained bydifferential scanning calorimetry, open cell ratio, average celldiameter, fluctuation in cell diameter, fluctuation in expansion ratioand water content were measured. The results are shown in Table 2.

EXAMPLES 9 TO 13

[0062] Pre-expanded particles were obtained in the same manner as inExample 8 except that the amount added of melamine (product name:Melamine, available from BASF Ltd.) and talc (average particle size 8μm) were the amount shown in Table 1 and properties were measured.Expansion conditions and results are shown in Table 2.

EXAMPLE 14

[0063] Resin particles and pre-expanded particles were obtained in thesame manner as in Example 8 except that 0.5% by weight of melamine(product name: Melamine, available from BASF Ltd.) as the triazinecompound and 0.45% by weight of mica (average particle size 8 μm) as theinorganic filler were used and properties were measured. Expansionconditions and results are shown in Table 2.

EXAMPLE 15

[0064] Resin particles and pre-expanded particles were obtained in thesame manner as in Example 8 except that 0.5% by weight of melamine(product name: Melamine, available from BASF Ltd.) as the triazinecompound and 0.45% by weight of kaoline (average particle size 0.4 μm)as the inorganic filler were used and properties were measured.Expansion conditions and results are shown in Table 2.

EXAMPLE 16

[0065] Resin particles and pre-expanded particles were obtained in thesame manner as in Example 8 except that 0.5% by weight of melamine(product name: Melamine, available from BASF Ltd.) as the triazinecompound and 0.45% by weight of refined bentonite (product name:BEN-GEL-23, available from Hojun Kogyo Co., Ltd.) as the inorganicfiller were used and properties were measured. Expansion conditions andresults are shown in Table 2.

EXAMPLE 17

[0066] Resin particles and pre-expanded particles were obtained in thesame manner as in Example 8 except that 0.5% by weight of melamine(product name: Melamine, available from BASF Ltd.) as the triazinecompound and 0.45% by weight of precipitated calcium carbonate (productname: Brilliant-1500, available from Shiraishi Kogyo Co., Ltd., averageparticle size 0.2 μm) as the inorganic filler were used and propertieswere measured. Expansion conditions and results are shown in Table 2.

EXAMPLE 18

[0067] Resin particles and pre-expanded particles were obtained in thesame manner as in Example 8 except that 0.5% by weight of melamine(product name: Melamine, available from BASF Ltd.) as the triazinecompound and 0.45% by weight of wet synthetic silica (product name:NIPGEL AZ-204, available from Nippon Silica Industrial Co., Ltd.,average particle size 1.3 μm) as the inorganic filler were used andproperties were measured. Expansion conditions and results are shown inTable 2.

EXAMPLE 19

[0068] Resin particles and pre-expanded particles were obtained in thesame manner as in Example 1 except that 1% by weight of isocyanuric acid(product name: NEO-CHLOR Cyanuric acid P, available from ShikokuChemicals Corporation, decomposition point: 330° C.) having a molecularweight of 129 per triazine skeleton unit was used as the triazinecompound and properties were measured. Expansion conditions and resultsare shown in Table 2.

EXAMPLES 20 TO 22

[0069] Pre-expanded particles were obtained in the same manner as inExample 8 except that isocyanuric acid (product name: NEO-CHLOR Cyanuricacid P, available from Shikoku Chemicals Corporation, decompositionpoint: 330° C.) as the triazine compound and talc (average particle size8 μm) as the inorganic filler were used in the amounts shown in Table 1and properties were measured. Expansion conditions and results are shownin Table 2.

EXAMPLE 23

[0070] Resin particles and pre-expanded particles were obtained in thesame manner as in Example 8 except that 1% by weight of melamineisocyanurate (available from Nissan Chemical Industries, Ltd., MC-440)which has a molecular weight of 115 per triazine skeleton unit as thetriazine compound and 0.45% by weight of talc (average particle size 8μm) as the inorganic filler were used and properties were measured.Expansion conditions and results are shown in Table 2.

EXAMPLE 24

[0071] Resin particles and pre-expanded particles were obtained in thesame manner as in Example 8 except that 1% by weight of acetoguanamine(available from Nippon Synthetic Chemical Industry Co., Ltd.) which hasa molecular weight of 125 per triazine skeleton unit as the triazinecompound and 0.45% by weight of talc (average particle size 8 μm) as theinorganic filler were used and properties were measured. Expansionconditions and results are shown in Table 2.

EXAMPLE 25

[0072] Resin particles and pre-expanded particles were obtained in thesame manner as in Example 8 except that propylene-butene-1 randomcopolymer (melting point 148° C., melting completion point 161° C., MIvalue 8 g/10 minutes) as the polypropylene resin, 0.5% by weight ofmelamine (product name: Melamine, available from BASF Ltd.) as thetriazine compound and 0.45% by weight of talc (average particle size 8μm) as the inorganic filler were used and properties were measured.Expansion conditions and results are shown in Table 2.

EXAMPLE 26

[0073] Resin particles and pre-expanded particles were obtained in thesame manner as in Example 8 except that ethylene-propylene-butene-1random copolymer (melting point 148° C., melting completion point 161°C., MI value 8 g/10 minutes) as the polypropylene resin, 0.5% by weightof melamine (product name: Melamine, available from BASF Ltd.) as thetriazine compound and 0.45% by weight of talc (average particle size 8μm) as the inorganic filler were used and properties were measured.Expansion conditions and results are shown in Table 2.

COMPARATIVE EXAMPLE 1

[0074] Resin particles and pre-expanded particles were obtained in thesame manner as in Example 1 except that 1% by weight of2,6-di-tert-4-(4,6-bis(octylthio)-1,3,5-triazine-ilamino)phenol (productname IRGANOX 565, available from Ciba Specialty Chemicals Co., Ltd.)which has a molecular weight of 589 per triazine skeleton unit was usedas the triazine compound and properties were measured. Expansionconditions and results are shown in Table 2.

COMPARATIVE EXAMPLE 2

[0075] Resin particles and pre-expanded particles were obtained in thesame manner as in Example 1 except that 1% by weight of1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl) 1,3,5-triazine-2,4,6 (1H,3H, 5H)-trion (product name IRGANOX 3114, available from Ciba SpecialtyChemicals Co., Ltd.) which has a molecular weight of 784 per triazineskeleton unit was used as the triazine compound and properties weremeasured. Expansion conditions and results are shown in Table 2.

COMPARATIVE EXAMPLE 3

[0076] Resin particles and pre-expanded particles were obtained in thesame manner as in Example 8 except that 1% by weight of2,6-di-tert-4-(4,6-bis(octylthio)-1,3,5-triazine-ilamino)phenol (productname IRGANOX 565, available from Ciba Specialty Chemicals Co., Ltd.)which has a molecular weight of 589 per triazine skeleton unit as thetriazine compound and 0.15% by weight of talc (average particle size 8μm) as the inorganic filler were used and properties were measured.Expansion conditions and results are shown in Table 2.

COMPARATIVE EXAMPLE 4

[0077] Resin particles and pre-expanded particles were obtained in thesame manner as in Example 8 except that 1% by weight of1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl) 1,3,5-triazine-2,4,6 (1H,3H, 5H)-trion (product name IRGANOX 3114, available from Ciba SpecialtyChemicals Co., Ltd.) which has a molecular weight of 784 per triazineskeleton unit as the triazine compound and 0.15% by weight of talc(average particle size 8 μm) as the inorganic filler were used andproperties were measured. Expansion conditions and results are shown inTable 2.

[0078] As indicated by Examples 1 to 7 and 19, when a polypropyleneresin composition which comprises ethylene-propylene random copolymer aspolypropylene resin (A) and a compound having a triazine skeleton andmolecular weight of at most 300 per triazine skeleton unit (B), is usedas the substrate resin, pre-expanded particles of the desired expansionratio can be obtained without worsening the open cell ratio and also,fluctuation in cell diameter and expansion ratio is small.

[0079] Furthermore, as indicated by Examples 8 to 18 and 20 to 24, whenusing in combination with a pre-determined amount of inorganic filler(D), pre-expanded particles having higher expansion ratio, smallfluctuation in cell diameter and expansion ratio can be obtained.

[0080] As indicated by Examples 25 and 26, in the case thatpropylene-butene-1 random copolymer and ethylene-propylene-butene-1random copolymer are used as polypropylene resin, pre-expanded particleswith small fluctuation in cell diameter and expansion ratio can beobtained.

[0081] On the other hand, as indicated by Comparative Examples 1 and 2,when a compound having a triazine skeleton and molecular weight of morethan 300 per triazine skeleton unit is added, pre-expanded particleswith high expansion ratio cannot be obtained and fluctuation in celldiameter and expansion ratio is large. Also, as indicated by ComparativeExamples 3 and 4, when a compound having a triazine skeleton andmolecular weight of more than 300 per triazine skeleton unit and aninorganic filler are added, though the expansion ratio becomes somewhatlarger, improvement in the fluctuation of cell diameter and expansionratio is not sufficient. TABLE 1 Polypropylene resin compositionTriazine compound Inorganic filler Amount of Ex. Amount Amount carbonblack No. Polypropylene resin Type (% by weight) Type (% by weight) (%by weight)  1 Ethylene-propylene Melamine 0.03 — — 2.5 random copolymer 2 Ethylene-propylene Melamine 0.08 — — 2.5 random copolymer  3Ethylene-propylene Melamine 0.5 — — 2.5 random copolymer  4Ethylene-propylene Melamine 1 — — 2.5 random copolymer  5Ethylene-propylene Melamine 3 — — 2.5 random copolymer  6Ethylene-propylene Melamine 5 — — 2.5 random copolymer  7Ethylene-propylene Melamine 10 — — 2.5 random copolymer  8Ethylene-propylene Melamine 0.08 Talc 0.15 2.5 random copolymer  9Ethylene-propylene Melamine 0.08 Talc 0.45 2.5 random copolymer 10Ethylene-propylene Melamine 0.5 Talc 0.15 2.5 random copolymer 11Ethylene-propylene Melamine 0.5 Talc 0.45 2.5 random copolymer 12Ethylene-propylene Melamine 0.5 Talc 1 2.5 random copolymer 13Ethylene-propylene Melamine 0.5 Talc 3 2.5 random copolymer 14Ethylene-propylene Melamine 0.5 Mica 0.45 2.5 random copolymer 15Ethylene-propylene Melamine 0.5 Kaoline 0.45 2.5 random copolymer 16Ethylene-propylene Melamine 0.5 Bentonite 0.45 2.5 random copolymer 17Ethylene-propylene Melamine 0.5 Calcium 0.45 2.5 random copolymercarbonate 18 Ethylene-propylene Melamine 0.5 Silica 0.45 2.5 randomcopolymer 19 Ethylene-propylene Isocyanuric acid 1 — — 2.5 randomcopolymer 20 Ethylene-propylene Isocyanuric acid 1 Talc 0.15 2.5 randomcopolymer 21 Ethylene-propylene Isocyanuric acid 1 Talc 0.45 2.5 randomcopolymer 22 Ethylene-propylene Isocyanuric acid 1 Talc 1 2.5 randomcopolymer 23 Ethylene-propylene Melamine isocyanurate 1 Talc 0.45 2.5random copolymer 24 Ethylene-propylene Acetoguanamine 1 Talc 0.45 2.5random copolymer 25 Propylene-butene-1 Melamine 0.5 Talc 0.45 2.5 randomcopolymer 26 Ethylene-propylene- Melamine 0.5 Talc 0.45 2.5 butene-1random copolymer Com. Ethylene-propylene IRGANOX 565 1 — — 2.5 Ex. 1random copolymer Com. Ethylene-propylene IRGANOX 3114 1 — — 2.5 Ex. 2random copolymer Com. Ethylene-propylene IRGANOX 565 1 Talc 0.15 2.5 Ex.3 random copolymer Com. Ethylene-propylene IRGANOX 3114 1 Talc 0.15 2.5Ex. 4 random copolymer

[0082] TABLE 2 Expansion condition Properties of pre-expanded particlesInner Pressure Open cell Average cell Fluctuation Fluctuation in WaterEx. temperature inside vessel Expansion Number ratio diameter in cellexpansion content No. (° C.) (MP a) ratio (times) of peaks (%) (μm)diameter ratio (%)  1 154 3.0 6 2 0.4 200 ◯ ⊚ 1.6  2 154 2.8 8 2 0.5 220⊚ ⊚ 2.1  3 154 2.8 12 2 0.4 270 ⊚ ⊚ 5.3  4 154 2.8 14 2 0.6 300 ⊚ ⊚ 6.8 5 154 2.8 16 2 0.8 340 ⊚ ⊚ 8.6  6 154 2.8 18 2 1.2 380 ⊚ ⊚ 9.5  7 1542.8 20 2 2.1 400 ◯ ◯ 10.6  8 154 2.8 12 2 0.6 160 ⊚ ⊚ 2.1  9 154 2.3 102 0.5 120 ⊚ ⊚ 2.0 10 154 2.8 16 2 0.5 240 ⊚ ⊚ 5.1 11 154 2.8 20 2 0.6200 ⊚ ⊚ 5.2 12 154 2.8 22 2 0.8 120 ⊚ ⊚ 5.2 13 154 2.8 24 2 1.8 80 ⊚ ⊚5.3 14 154 2.8 18 2 0.5 220 ⊚ ⊚ 5.2 15 154 2.8 17 2 0.4 230 ⊚ ⊚ 5.1 16154 2.8 17 2 0.6 230 ⊚ ⊚ 5.3 17 154 2.8 18 2 0.4 220 ⊚ ⊚ 5.1 18 154 2.819 2 0.4 210 ⊚ ⊚ 5.2 19 154 3.0 8 2 0.5 180 ⊚ ⊚ 6.9 20 154 3.0 13 2 0.6160 ⊚ ⊚ 7.1 21 154 2.0 10 2 0.5 120 ⊚ ⊚ 7.0 22 154 3.0 17 2 1.0 90 ⊚ ⊚7.1 23 154 2.8 13 2 0.5 190 ⊚ ⊚ 5.3 24 154 2.8 12 2 0.4 160 ⊚ ⊚ 4.6 25157 2.8 20 2 0.4 200 ⊚ ⊚ 5.1 26 157 2.8 20 2 0.4 210 ⊚ ⊚ 5.2 Com. 1542.8 2 2 0.5 380 X X 0.3 Ex. 1 Com. 154 2.8 2 2 0.4 360 X X 0.4 Ex. 2Com. 154 2.8 4 2 0.5 210 Δ Δ 0.6 Ex. 3 Com. 154 2.8 4 2 0.6 220 Δ Δ 0.6Ex. 4

[0083] The composition of the polypropylene resin composition ofExamples 27 to 60 and Comparative Examples 5 to 13 are shown in Table 3and the expansion conditions and properties of the obtained pre-expandedparticles are shown in Table 4.

EXAMPLE 27

[0084] 100 parts by weight of ethylene-propylene random copolymer(melting point 146° C., melting completion point 160° C., MI value 9g/10 minutes), 2 parts by weight of ethylene ionomer resin (productname: Himilan SD 100, available from Mitsui Dupont Poly Chemical Co.,Ltd.) obtained by crosslinking ethylene-(meth)acrylic acid copolymerwith a potassium ion as the hydrophilic polymer, 1 part by weight ofisocyanuric acid (product name: NEO-CHLOR Cyanuric acid P, availablefrom Shikoku Chemicals Corporation) having a molecular weight of 129 pertriazine skeleton unit as the triazine compound and 2.6 parts by weightof carbon black as the coloring agent were compounded. The mixture wasmelt mixed in a 50 mmφ single screw extruder, extruded into strands froma cylindrical die of 2.2 mmφ in diameter, cooled with water and cut witha cutter to obtain resin particles of 1.8 mg/particle.

[0085] A 0.35 m³ autoclave was charged with 100 parts by weight of theobtained resin particles (65 kg), 200 parts by weight of water, 0.5 partby weight of basic calcium tertiary phosphate and 0.01 part by weight ofsodium n-paraffinsulfonate and while agitating, the contents of theautoclave were heated to the inner temperature shown in Table 4. Then,the pressure within the autoclave was increased by compressed air to thepressure inside the vessel shown in Table 4 and the temperature withinthe vessel was maintained for 30 minutes. A valve located at the lowerpart of the autoclave was opened and, the contents of the autoclave weredischarged into saturated vapor atmosphere of 100° C. through an openorifice of 3.2 mmφ to obtain pre-expanded particles.

[0086] As properties of the obtained pre-expanded particles, expansionratio, the number of melting peaks in the DSC curve obtained bydifferential scanning calorimetry, open cell ratio, average celldiameter, fluctuation in cell diameter and fluctuation in expansionratio were measured. The results are shown in Table 4.

EXAMPLE 28

[0087] 100 parts by weight of ethylene-propylene random copolymer(melting point 146° C., melting completion point 160° C., MI value 9g/10 minutes), 2 parts by weight of ethylene ionomer resin (productname: Himilan SD 100, available from Mitsui Dupont Poly Chemical Co.,Ltd.) obtained by crosslinking ethylene-(meth)acrylic acid copolymerwith a potassium ion as the hydrophilic polymer, 1 part by weight ofisocyanuric acid (product name: NEO-CHLOR Cyanuric acid P, availablefrom Shikoku Chemicals Corporation) having a molecular weight of 129 pertriazine skeleton unit as the triazine compound, 0.15 part by weight oftalc (average particle size 8 μm) and 2.6 parts by weight of carbonblack as the coloring agent were compounded. The mixture was melt mixedin a 50 mmφ single screw extruder, extruded into strands from acylindrical die of 2.2 mmφ in diameter, cooled with water and cut with acutter to obtain resin particles of 1.8 mg/particle.

[0088] Pre-expanded particles were obtained and properties were measuredin the same manner as in Example 27. The results are shown in Table 4.

EXAMPLES 29 TO 33

[0089] Resin particles and pre-expanded particles were obtained in thesame manner as in Example 28 except that the amount to be added ofethylene ionomer resin (product name: Himilan SD 100, available fromMitsui Dupont Poly Chemical Co., Ltd.) obtained by crosslinkingethylene-(meth)acrylic acid copolymer with a potassium ion, isocyanuricacid (product name: NEO-CHLOR Cyanuric acid P, available from ShikokuChemicals Corporation) and talc (average particle size 8 μm) were theamount shown in Table 3 and properties were measured. The results areshown in Table 4.

EXAMPLE 34

[0090] Resin particles and pre-expanded particles were obtained in thesame manner as in Example 33 except that mica (average particle size 8μm) was used as the inorganic filler and properties were measured. Theresults are shown in Table 4.

EXAMPLE 35

[0091] Resin particles and pre-expanded particles were obtained in thesame manner as in Example 33 except that kaoline (average particle size0.4 μm) was used as the inorganic filler and properties were measured.The results are shown in Table 4.

EXAMPLE 36

[0092] Resin particles and pre-expanded particles were obtained in thesame manner as in Example 33 except that refined bentonite (productname: BEN-GEL-23, available from Hojun Kogyo Co., Ltd.) was used as theinorganic filler and properties were measured. The results are shown inTable 4.

EXAMPLE 37

[0093] Resin particles and pre-expanded particles were obtained in thesame manner as in Example 33 except that laponite (product name:Laponite XLG, available from Nippon Silica Industrial Co., Ltd.) wasused as the inorganic filler and properties were measured. The resultsare shown in Table 4.

EXAMPLE 38

[0094] Resin particles and pre-expanded particles were obtained in thesame manner as in Example 33 except that wet synthetic silica (productname: NIPGEL AZ-204, available from Nippon Silica Industrial Co., Ltd.,average particle size 1.3 μm) was used as the inorganic filler andproperties were measured. The results are shown in Table 4.

EXAMPLE 39

[0095] Resin particles and pre-expanded particles were obtained in thesame manner as in Example 28 except that precipitated calcium carbonate(product name: Brilliant-1500, available from Shiraishi Kogyo Co., Ltd.,average particle size 0.2 μm) was used as the inorganic filler andproperties were measured. The results are shown in Table 4.

EXAMPLES 40 TO 42

[0096] Resin particles and pre-expanded particles were obtained in thesame manner as in Example 28 except that ethylene ionomer resin (productname: Himilan 1707, available from Mitsui Dupont Poly Chemical Co.,Ltd.) obtained by crosslinking ethylene-(meth)acrylic acid copolymerwith a sodium ion was added as the hydrophilic polymer in the amountshown in Table 3 and properties were measured. The results are shown inTable 4.

EXAMPLE 43

[0097] Resin particles and pre-expanded particles were obtained in thesame manner as in Example 27 except that melamine (product name:Melamine, available from BASF Ltd., decomposition point: 300° C.) whichhas a molecular weight of 126 per triazine skeleton unit, was used asthe triazine compound and properties were measured. The results areshown in Table 4.

EXAMPLE 44

[0098] Resin particles and pre-expanded particles were obtained in thesame manner as in Example 28 except that melamine (product name:Melamine, available from BASF Ltd., decomposition point: 300° C.) whichhas a molecular weight of 126 per triazine skeleton unit, was used asthe triazine compound and properties were measured. The results areshown in Table 4.

EXAMPLES 45 TO 54

[0099] Resin particles and pre-expanded particles were obtained in thesame manner as in Example 44 except that the amount to be added ofethylene ionomer resin (product name: Himilan SD 100, available fromMitsui Dupont Poly Chemical Co., Ltd.) obtained by crosslinkingethylene-(meth)acrylic acid copolymer with a potassium ion and melamine(product name: Melamine, available from BASF Ltd.) were the amount shownin Table 3 and properties were measured. The results are shown in Table4.

EXAMPLE 55

[0100] Resin particles and pre-expanded particles were obtained in thesame manner as in Example 44 except that ethylene ionomer resin (productname: Himilan 1707, available from Mitsui Dupont Poly Chemical Co.,Ltd.) obtained by crosslinking ethylene-(meth)acrylic acid copolymerwith a sodium ion was used as the hydrophilic polymer and propertieswere measured. The results are shown in Table 4.

EXAMPLE 56

[0101] Resin particles and pre-expanded particles were obtained in thesame manner as in Example 44 except that propylene-butene-1 randomcopolymer (melting point 148° C., melting completion point 161° C., MIvalue 8 g/10 minutes) was used as the polypropylene resin and propertieswere measured. The results are shown in Table 4.

EXAMPLE 57

[0102] Resin particles and pre-expanded particles were obtained in thesame manner as in Example 44 except that ethylene-propylene-butene-1random copolymer (melting point 148° C., melting completion point 161°C., MI value 8 g/10 minutes) was used as the polypropylene resin andproperties were measured. The results are shown in Table 4.

EXAMPLE 58

[0103] Resin particles and pre-expanded particles were obtained in thesame manner as in Example 44 except that melamine isocyanurate (productname: Melamine, available from BASF Ltd.) which has a molecular weightof 113 per triazine skeleton unit, was used as the triazine compound andproperties were measured. The results are shown in Table 4.

EXAMPLE 59

[0104] Resin particles and pre-expanded particles were obtained in thesame manner as in Example 28 except that isocyanuric acid (product name:NEO-CHLOR Cyanuric acid P, available from Shikoku Chemicals Corporation)which has a molecular weight of 129 per triazine skeleton unit, was usedas the triazine compound in the amount shown in Table 3 and propertieswere measured. The results are shown in Table 4.

EXAMPLE 60

[0105] Resin particles and pre-expanded particles were obtained in thesame manner as in Example 28 except that melamine (product name:Melamine, available from BASF Ltd., decomposition point: 300° C.) whichhas a molecular weight of 126 per triazine skeleton unit, was used asthe triazine compound in the amount shown in Table 3 and properties weremeasured. The results are shown in Table 4.

COMPARATIVE EXAMPLE 5

[0106] 100 parts by weight of ethylene-propylene random copolymer(melting point 146° C., melting completion point 160° C., MI value 9g/10 minutes), 2 parts by weight of ethylene ionomer resin (productname: Himilan SD 100, available from Mitsui Dupont Poly Chemical Co.,Ltd.) obtained by crosslinking ethylene-(meth)acrylic acid copolymerwith a potassium ion as the hydrophilic polymer, 0.15 part by weight oftalc (average particle size 8 μm) as the inorganic filler and 2.6 partsby weight of carbon black were compounded. The mixture was melt mixed ina 50 mmφ single screw extruder, extruded into strands from a cylindricaldie of 2.2 mmφ in diameter, cooled with water and cut with a cutter toobtain resin particles of 1.8 mg/particle.

[0107] Pre-expanded particles were obtained in the same manner as inExample 27 under the expansion conditions shown in Table 4 andproperties were measured. The results are shown in Table 4.

COMPARATIVE EXAMPLE 6

[0108] Resin particles and pre-expanded particles were obtained in thesame manner as in Comparative Example 5 except that mica (averageparticle size 8 μm) was used as the inorganic filler and properties weremeasured. The results are shown in Table 4.

COMPARATIVE EXAMPLE 7

[0109] Resin particles and pre-expanded particles were obtained in thesame manner as in Comparative Example 5 except that kaoline (averageparticle size 0.4 μm) was used as the inorganic filler and propertieswere measured. The results are shown in Table 4.

COMPARATIVE EXAMPLE 8

[0110] Resin particles and pre-expanded particles were obtained in thesame manner as in Comparative Example 5 except that refined bentonite(product name: BEN-GEL-23, available from Hojun Kogyo Co., Ltd.) wasused as the inorganic filler and properties were measured. The resultsare shown in Table 4.

COMPARATIVE EXAMPLE 9

[0111] Resin particles and pre-expanded particles were obtained in thesame manner as in Comparative Example 5 except that ethylene ionomerresin (product name: Himilan 1707, available from Mitsui Dupont PolyChemical Co., Ltd.) obtained by crosslinking ethylene-(meth)acrylic acidcopolymer with a sodium ion was used as the hydrophilic polymer andproperties were measured. The results are shown in Table 4.

COMPARATIVE EXAMPLE 10

[0112] Resin particles and pre-expanded particles were obtained in thesame manner as in Comparative Example 5 except that propylene-butene-1random copolymer (melting point 148° C., melting completion point 161°C., MI value 8 g/10 minutes) was used as the olypropylene resin andproperties were measured. The results are shown in Table 4.

COMPARATIVE EXAMPLE 11

[0113] Resin particles and pre-expanded particles were obtained in thesame manner as in Comparative Example 5 except thatethylene-propylene-butene-1 random copolymer (melting point 148° C.,melting completion point 161° C., MI value 8 g/10 minutes) was used asthe polypropylene resin and properties were measured. The results areshown in Table 4.

COMPARATIVE EXAMPLE 12

[0114] Resin particles and pre-expanded particles were obtained in thesame manner as in Example 28 except that2,6-di-tert-4-(4,6-bis(octylthio)-1,3,5-triazine-ilamino)phenol (productname IRGANOX 565, available from Ciba Specialty Chemicals Co., Ltd.)which has a molecular weight of 589 per triazine skeleton unit was usedinstead of isocyanuric acid and properties were measured. The resultsare shown in Table 4.

COMPARATIVE EXAMPLE 13

[0115] Resin particles and pre-expanded particles were obtained in thesame manner as in Example 28 except that1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl)1,3,5-triazine-2,4,6(1H,3H,5H)-trion (product name IRGANOX 3114,available from Ciba Specialty Chemicals Co., Ltd.) which has a molecularweight of 784 per triazine skeleton unit was used instead of isocyanuricacid and properties were measured. The results are shown in Table 4.

[0116] As indicated by Examples 27 to. 33, 40 to 55, 58 to 60, in thecase that the pre-expanded particles are prepared from polypropyleneresin containing 100 parts by weight of ethylene-propylene randomcopolymer as polypropylene resin (A), 0.05 to 10 parts by weight ofisocyanuric acid, melamine or melamine isocyanurate as the compoundhaving a triazine skeleton and molecular weight of at most 300 pertriazine skeleton unit (B), 0.01 to 20 parts by weight of ethyleneionomer resin prepared by crosslinking ethylene-(meth)acrylic acidcopolymer with an ion as the hydrophilic polymer (C) and 0 to 10 partsby weight of talc as the inorganic filler (D), pre-expanded particles ofthe desired expansion ratio can be obtained and fluctuation in celldiameter and expansion ratio is small.

[0117] Also, as indicated by Examples 27 to 33, 40 to 55 and 58, byadding 0.05 to 6 parts by weight of isocyanuric acid, melamine ormelamine isocyanurate as the compound having a triazine skeleton andmolecular weight of at most 300 per triazine skeleton unit (B), not onlycan fluctuation in cell diameter and expansion ratio of the obtainedpre-expanded particles be kept small, but also the open cell ratio canbe decreased.

[0118] Furthermore, as indicated in Examples 36 to 39, in the case thatmica, kaoline, bentonite, laponite, silica or calcium carbonate is usedas the inorganic filler, pre-expanded particles with small fluctuationin cell diameter and expansion ratio can be obtained. In the samemanner, as indicated in Examples 56 and 57, in the case thatpropylene-butene-1 random copolymer or ethylene-propylene-butene-1random copolymer is used as the polypropylene resin, pre-expandedparticles with small fluctuation in cell diameter and expansion ratiocan be obtained.

[0119] On the other hand, as indicated in Comparative Examples 5 to 11,in the case that a compound having a triazine skeleton and molecularweight of at most 300 per triazine skeleton unit is not added, thefluctuation in cell diameter and expansion ratio of the obtainedpre-expanded particles is large in comparison to Examples 27 to 60.

[0120] In addition, as indicated by Comparative Examples 12 and 13, inthe case that a compound having a triazine skeleton and molecular weightof more than 300 per triazine skeleton unit is added, the fluctuation incell diameter and expansion ratio of the obtained pre-expanded particlesis large in comparison to Examples 27 to 60. TABLE 3 Polypropylene resincomposition Hydrophilic Triazine compound Amount of polymer MolecularInorganic filler carbon Ex. Amount weight per Amount Amount black No.Polypropylene resin Type (part) Type triazine unit (part) Type (part)(part) 27 Ethylene-propylene SD100 2.0 Isocyanuric 129 1.0 — — 2.6random copolymer acid 28 Ethylene-propylene SD100 2.0 Isocyanuric 1291.0 Talc 0.15 2.6 random copolymer acid 29 Ethylene-propylene SD100 2.0Isocyanuric 129 1.0 Talc 0.01 2.6 random copolymer acid 30Ethylene-propylene SD100 2.0 Isocyanuric 129 0.5 Talc 0.15 2.6 randomcopolymer acid 31 Ethylene-propylene SD100 2.0 Isocyanuric 129 0.7 Talc0.15 2.6 random copolymer acid 32 Ethylene-propylene SD100 0.5Isocyanuric 129 1.0 Talc 0.15 2.6 random copolymer acid 33Ethylene-propylene SD100 1.0 Isocyanuric 129 1.0 Talc 0.15 2.6 randomcopolymer acid 34 Ethylene-propylene SD100 1.0 Isocyanuric 129 1.0 Mica0.15 2.6 random copolymer acid 35 Ethylene-propylene SD100 1.0Isocyanuric 129 1.0 Kaoline 0.15 2.6 random copolymer acid 36Ethylene-propylene SD100 1.0 Isocyanuric 129 1.0 Bentonite 0.15 2.6random copolymer acid 37 Ethylene-propylene SD100 1.0 Isocyanuric 1291.0 Laponite 0.15 2.6 random copolymer acid 38 Ethylene-propylene SD1001.0 Isocyanuric 129 1.0 Silica 0.15 2.6 random copolymer acid 39Ethylene-propylene SD100 2.0 Isocyanuric 129 1.0 Calcium 0.15 2.6 randomcopolymer acid carbonate 40 Ethylene-propylene 1707 0.5 Isocyanuric 1291.0 Talc 0.15 2.6 random copolymer acid 41 Ethylene-propylene 1707 1.0Isocyanuric 129 1.0 Talc 0.15 2.6 random copolymer acid 42Ethylene-propylene 1707 2.0 Isocyanuric 129 1.0 Talc 0.15 2.6 randomcopolymer acid 43 Ethylene-propylene SD100 2.0 Melamine 126 1.0 — — 2.6random copolymer 44 Ethylene-propylene SD100 2.0 Melamine 126 1.0 Talc0.15 2.6 random copolymer 45 Ethylene-propylene SD100 2.0 Melamine 1260.06 Talc 0.15 2.6 random copolymer 46 Ethylene-propylene SD100 2.0Melamine 126 0.1 Talc 0.15 2.6 random copolymer 47 Ethylene-propyleneSD100 2.0 Melamine 126 0.5 Talc 0.15 2.6 random copolymer 48Ethylene-propylene SD100 2.0 Melamine 126 3.0 Talc 0.15 2.6 randomcopolymer 49 Ethylene-propylene SD100 2.0 Melamine 126 4.0 Talc 0.15 2.6random copolymer 50 Ethylene-propylene SD100 0.05 Melamine 126 1.0 Talc0.15 2.6 random copolymer 51 Ethylene-propylene SD100 0.1 Melamine 1261.0 Talc 0.15 2.6 random copolymer 52 Ethylene-propylene SD100 3.0Melamine 126 1.0 Talc 0.15 2.6 random copolymer 53 Ethylene-propyleneSD100 5.0 Melamine 126 1.0 Talc 0.15 2.6 random copolymer 54Ethylene-propylene SD100 8.0 Melamine 126 1.0 Talc 0.15 2.6 randomcopolymer 55 Ethylene-propylene SD100 2.0 Melamine 126 1.0 Talc 0.15 2.6random copolymer 56 Propylene-butene-1 SD100 2.0 Melamine 126 1.0 Talc0.15 2.6 random copolymer 57 Ethylene-propylene- SD100 2.0 Melamine 1261.0 Talc 0.15 2.6 butene-1 random copolymer 58 Ethylene-propylene SD1002.0 Melamine 113 1.0 Talc 0.15 2.6 random copolymer isocyanurate 59Ethylene-propylene SD100 2.0 Isocyanuric 129 10.0 Talc 0.15 2.6 randomcopolymer acid 60 Ethylene-propylene SD100 2.0 Melamine 126 10.0 Talc0.15 2.6 random copolymer 59 Ethylene-propylene SD100 2.0 Isocyanuric129 10.0 Talc 0.15 2.6 random copolymer acid 60 Ethylene-propylene SD1002.0 Melamine 126 10.0 Talc 0.15 2.6 random copolymer Com.Ethylene-propylene SD100 2.0 — — — Talc 0.15 2.6 Ex. 5 random copolymerCom. Ethylene-propylene SD100 2.0 — — — Mica 0.15 2.6 Ex. 6 randomcopolymer Com. Ethylene-propylene SD100 2.0 — — — Kaoline 0.15 2.6 Ex. 7random copolymer Com. Ethylene-propylene SD100 2.0 — — — Bentonite 0.152.6 Ex. 8 random copolymer Com. Ethylene-propylene 1707 2.0 — — — Talc0.15 2.6 Ex. 9 random copolymer Com. Propylene-butene-1 SD100 2.0 — — —Talc 0.15 2.6 Ex. 10 random copolymer Com. Ethylene-propylene SD100 2.0— — — Talc 0.15 2.6 Ex. 11 butene-1 random copolymer Com.Ethylene-propylene SD100 2.0 IRGANOX565 589 1.0 Talc 0.15 2.6 Ex. 12random copolymer Com. Ethylene-propylene SD100 2.0 IRGANOX3114 784 1.0Talc 0.15 2.6 Ex. 13 random copolymer

[0121] TABLE 4 Expansion condition Inner Pressure Properties ofpre-expanded particles Ex. temperature inside vessel Expansion NumberOpen cell Average cell Fluctuation in Fluctuation in No. (° C.) (MP a)ratio (times) of peaks ratio (%) diameter (μm) cell diameter expansionratio 27 156 2.6 11 2 3 220 ⊚ ⊚ 28 156 2.6 16 2 2 180 ⊚ ⊚ 29 156 2.6 142 3 200 ⊚ ⊚ 30 156 2.6 15 2 2 180 ⊚ ⊚ 31 156 2.6 15 2 2 180 ⊚ ⊚ 32 1562.6 13 2 3 170 ⊚ ⊚ 33 156 2.6 14 2 2 180 ⊚ ⊚ 34 156 2.6 12 2 3 240 ⊚ ⊚35 156 2.6 12 2 2 240 ⊚ ⊚ 36 156 2.6 12 2 2 260 ⊚ ⊚ 37 156 2.6 12 2 3240 ⊚ ⊚ 38 156 2.6 12 2 3 260 ⊚ ⊚ 39 156 2.6 18 2 3 210 ⊚ ⊚ 40 156 2.6 92 3 160 ⊚ ⊚ 41 156 2.6 10 2 3 160 ⊚ ⊚ 42 156 2.6 12 2 3 150 ⊚ ⊚ 43 1562.6 12 2 2 300 ⊚ ⊚ 44 156 2.6 16 2 3 260 ⊚ ⊚ 45 156 2.6 9 2 2 350 ◯ ◯ 46156 2.6 10 2 2 320 ⊚ ⊚ 47 156 2.6 11 2 2 300 ⊚ ⊚ 48 156 2.6 20 2 4 170 ⊚⊚ 49 156 2.6 21 2 6 100 ⊚ ⊚ 50 156 2.6 10 2 2 320 ◯ ◯ 51 156 2.6 11 2 3300 ⊚ ⊚ 52 156 2.6 19 2 4 220 ⊚ ⊚ 53 156 2.6 20 2 4 180 ⊚ ⊚ 54 156 2.622 2 5 120 ⊚ ◯ 55 156 2.6 9 2 3 220 ⊚ ⊚ 56 157 2.7 16 2 2 280 ⊚ ⊚ 57 1572.7 16 2 2 260 ⊚ ⊚ 58 156 2.6 13 2 3 180 ⊚ ⊚ 59 156 2.6 22 2 8 80 ⊚ ◯ 60156 2.6 23 2 8 80 ⊚ ◯ Com. 156 2.6 12 2 3 170 Δ Δ Ex. 5 Com. 156 2.6 102 2 200 X X Ex. 6 Com. 156 2.6 10 2 3 190 X X Ex. 7 Com. 156 2.6 10 2 3210 Δ Δ Ex. 8 Com. 156 2.6 7 2 2 130 Δ X Ex. 9 Com. 156 2.6 6 2 2 120 ΔX Ex. 10 Com. 156 2.6 6 2 2 130 Δ X Ex. 11 Com. 156 2.6 12 2 2 180 Δ ΔEx. 12 Com. 156 2.6 13 2 2 160 Δ Δ Ex. 13

[0122] The method by which the properties of the pre-expanded particleswere evaluated is indicated below.

[0123] (Expansion Ratio)

[0124] After measuring the weight of the pre-expanded particles, thevolume was measured by impregnating with ethanol in a 100 mlmesscylinder and the true density value was found. The expansion ratiowas calculated by dividing the density of the resin particles of thepolypropylene resin composition by the true density value.

[0125] (Number of Melting Peaks in the DSC Curve Obtained byDifferential Scanning Calorimetry)

[0126] The number of melting peaks were read from the DSC curve obtainedwhen 1 to 10 mg of pre-expanded particles were heated from 40° C. to220° C. at a heating rate of 10° C./minute using a differential scanningcalorimeter.

[0127] (Open Cell Ratio)

[0128] The closed cell volume (V₀) of the pre-expanded particles wasfound using an air comparison pychnometer (930 model, made by BeckmanCo., Ltd.), the ethanol impregnated volume (V₁) of the same samples wasadditionally found, and the open cell ratio was found from

Open cell ratio (%)=((V ₁ −V ₀)/V ₁)×100.

[0129] As the open cell ratio increases, decrease in moldability whenconducting in-mold expansion molding of the pre-expanded particles andmechanical strength such as compression strength when made into anin-mold expanded article is caused. In order to prevent significantdecrease in moldability and mechanical strength, the open cell ratio ispreferably at most 6%.

[0130] (Average Cell Diameter)

[0131] 30 pre-expanded particles were randomly picked out of theobtained pre-expanded particles, the cell diameter was measuredaccording to JIS K6402 and the average cell diameter (d) was found. Theaverage cell diameter is preferably approximately 100 to 500 μm in viewof moldability when conducting in-mold expansion molding of thepre-expanded particles and color when made into an in-mold expandedarticle. When made microscopic to less than 50 μm, moldability whenconducting in-mold expansion molding tends to decrease.

[0132] (Fluctuation in Cell Diameter)

[0133] The ratio (cell diameter fluctuation U) of the standard deviation(a) which represents fluctuation in cell diameter and the average celldiameter (d) was found by

U(%)=(σ/d)×100.

[0134] The smaller U is, the more uniform the cells are. In most cases,when the value of U is at least 20%, unevenness in color tends to becomenoticeable and as the value of U becomes larger, unevenness in colortends to increase. The value of U was classified and evaluation wasconducted according to the following criteria.

[0135] ⊚: value of U is less than 10%

[0136] ◯: value of U is at least 10% and less than 20%

[0137] Δ: value of U is at least 20% and less than 35%

[0138] X: value of U is at least 35%

[0139] (Fluctuation in Expansion Ratio)

[0140] 0.3 to 1 L of the obtained pre-expanded particles were sieved bya JIS Z8801 standard sieve (8 kinds of 3.5, 4, 5, 6, 7, 8, 9 and 10mesh) and from the weight ratio W_(i) and expansion ratio K_(i) of thepre-expanded particles remaining in the sieve, the weighted expansionaverage ratio K_(av) and expansion ratio standard deviation σ_(m) wascalculated by

K _(av)=Σ(K _(i) ×W _(i))

σ_(m) ={square root}{square root over ( )}[Σ{W _(i)×(K _(av) −K_(i))²}].

[0141] Using these values, expansion ratio fluctuation V was calculatedby

V(%)=(σ_(m) /K _(av))×100.

[0142] The smaller V is, the smaller the fluctuation in expansion ratiois.

[0143] The value of V was classified and evaluation was conductedaccording to the following criteria.

[0144] ⊚: value of V is less than 7.5%

[0145] ◯: value of V is at least 7.5% and less than 10%

[0146] Δ: value of V is at least 10% and less than 12.5%

[0147] X: value of V is at least 12.5% and less than 15%

[0148] XX: value of V is at least 15%

[0149] (Water Content of the Pre-Expanded Particles)

[0150] After dehydrating the surface of the pre-expanded particlesimmediately after pre-expansion by an airflow, the weight (W1) wasmeasured. Furthermore, the weight (W2) after drying the pre-expandedparticles for 12 hours in an oven at 80° C. was measured and the watercontent was calculated from

Water content (%)=(W1−W2)/W2×100.

[0151] Industrial Applicability

[0152] Polypropylene resin pre-expanded particles which have as a baseresin, a polypropylene resin composition containing 100 parts by weightof polypropylene resin (A) and a compound having a triazine skeleton andmolecular weight of at most 300 per triazine skeleton unit (B) and areprepared with water as the foaming agent, have small fluctuation inexpansion ratio and cell diameter and decreased fluctuation in weightand unevenness in color when made into an in-mold expanded article.Furthermore, by including hydrophilic polymer (C) and/or inorganicfiller (D), the expansion ratio improves further.

1. A polypropylene resin pre-expanded particle comprising as a baseresin a polypropylene resin composition containing: (A) a polypropyleneresin and (B) a compound having a triazine skeleton and molecular weightof at most 300 per triazine skeleton unit.
 2. The polypropylene resinpre-expanded particle of claim 1, which further comprises hydrophilicpolymer (C).
 3. The polypropylene resin pre-expanded particle of claim 1or 2, wherein said compound (B) having a triazine skeleton and molecularweight of at most 300 per triazine skeleton unit is contained in anamount of 0.05 to 6 parts by weight based on 100 parts by weight of saidpolypropylene resin (A).
 4. The polypropylene resin pre-expandedparticle of claim 2, wherein said hydrophilic polymer (C) is containedin an amount of 0.01 to 20 parts by weight based on 100 parts by weightof said polypropylene resin (A).
 5. The polypropylene resin pre-expandedparticle of claim 1 or 2, which further comprises inorganic filler (D).6. The polypropylene resin pre-expanded particle of claim 5, whereinsaid inorganic filler (D) is contained in an amount of 0.005 to 10 partsby weight based on 100 parts by weight of said polypropylene resin (A).7. The polypropylene resin pre-expanded particle of claim 1 or 2,wherein said polypropylene resin (A) is at least one member selectedfrom the group consisting of ethylene-propylene random copolymer,propylene-butene-1 random copolymer and ethylene-propylene-butene-1random copolymer.
 8. The polypropylene resin pre-expanded particle ofclaim 1 or 2, wherein said compound (B) having a triazine skeleton andmolecular weight of at most 300 per triazine skeleton unit is at leastone member selected from the group consisting of melamine, isocyanuricacid and melamine isocyanurate.
 9. The polypropylene resin pre-expandedparticle of claim 2, wherein said hydrophilic polymer (C) is an ethyleneionomer resin obtained by crosslinking an ethylene-(meth)acrylic acidcopolymer with an alkali metal ion.
 10. The polypropylene resinpre-expanded particle of claim 1 or 2, which has two melting peaks inthe DSC curve obtained by differential scanning calorimetry.
 11. Anin-mold expanded article comprising the polypropylene resin pre-expandedparticle of claim 1 or 2.